Chains with conventionally sintered oil-retaining bushings are widely used because periodic lubrication is not required. However, if the oil-retaining ratio is increased to enhance the lubrication property by lowering the sintered of the bushing, the mechanical strength of the bushing is lowered. On the other hand, if the mechanical strength is improved by increasing the sintered density, the oil-retaining ratio is lowered, and the bushing exhibits a rather poor lubrication property. Thus, it has been the conventional wisdom in the art to increase the oil-retaining capacity by lowering the sintered density; and at the same time, to retain the mechanical strength by making the diameter of the bushing larger.
In case of the above-mentioned prior art, as shown in FIG. 2, when the teeth of a sprocket engage the sintered oil-retaining bushing B, the bushing B is unable to freely rotate, since its both ends are force-fit into the bores of the inner link plates L1. Consequently, only a limited portion of each bushing B is subjected to wear due to the engagement with the sprocket. In other words, the wear and stress is highly localized and especially damaging. Likewise, the teeth of the sprocket are subjected to localized wear due to slippage, which tends to give rise to improper engagement between the sprocket and chain.
Furthermore, if a roller is used in conjunction with the bushing in order to prevent such localized wear, the outer diameter of the roller is very large. because the outer diameter of the bushing itself is large to retain mechanical strength, as mentioned above. This in turn requires the engagement pitch with the sprocket being unduly large, which requires the lengths of link plates to become larger. The result has been a practical inability to produce strong, self-lubricating small-sized chains.
Accordingly, it has become accepted practice in the art to make small-sized chains, with bushings by means of direct contact between the bushings and sprockets, that is, without rollers. Therefore, localized wear and premature failure of such chains has been unavoidable.